The present invention relates generally to protective coatings for semiconductor devices, and more particularly to the surface structure of solid-state fingerprint sensors.
A capacitive fingerprint sensor is disclosed in commonly-assigned U.S. Pat. No. 6,114,862 by Tartagni et al., the disclosure of which is hereby incorporated by reference. The Tartagni et al. patent discloses a two-dimensional array of capacitive sensor cells or xe2x80x9cpixelxe2x80x9d cells. The array includes a dielectric layer that forms a sensing surface for contact with the fingerprint-bearing skin of a user""s finger. Capacitor plates at each pixel location beneath the dielectric layer distinguish between the presence of fingerprint ridges and valleys above the sensing surface during a fingerprint sensing operation.
The dielectric layer that forms the sensing surface in prior art fingerprint sensors, such as that disclosed in the Tartagni et al. patent, is typically an ultra-hard material that is resistive to wear from repeated use. One example of an ultra-hard dielectric material used in prior art fingerprint sensors is a layer of silicon carbide. It has been found that such ultra-hard dielectric layers can be damaged by impact with sharp instruments, such as the point of a ballpoint pen. Although it is impractical to design a product that will survive intentional destructive misuse, it is desirable to design a product that will survive occasional accidental misuse. Therefore, it would be desirable to find a material for use as a protective coating for a fingerprint sensor that is durable with normal expected use, resistive to occasional accidental misuse, and effective in permitting accurate sensing of a fingerprint of a finger applied to a sensing surface.
The present invention provides a protective coating for a fingerprint sensor that comprises a polymeric material with conductive particles suspended therein. The protective coating has an exposed upper surface for contact with the fingerprint-bearing skin of a user""s finger. The protective coating is durable and resistive to damage from scratching or impact from a sharp or pointed object. The protective coating is applied to an underlying hard dielectric body that includes capacitive sensing plates embedded therein. The conductive particles suspended in the protective coating are generally uniformly distributed and are smaller than the thickness of the protective coating so that they define parallel strings of series capacitors between the upper sensing surface of the protective coating and the capacitive sensing plates disposed beneath the protective coating at each pixel location.